Lou Reed struggled to deal with criticism over his recent collaboration with Metallica as he took fans’ negative comments “personally”, according to Lars Ulrich. The veteran musician teamed up with the heavy rockers to record 2011’s Lulu, which debuted at number 36 on the Billboard 200 and scored the lowest ever first-week sales for a Metallica album.

The record also received harsh words from music critics and fans, with Reed revealing he had been sent a barrage of “threatening” letters from fuming Metallica followers.

Reed shrugged off the criticism and insisted he was “in this for the fun of it,” but Ulrich has now revealed the star was hurt by the nasty comments.

He tells radio station DC101, “Obviously, it’s fantastic in 2012 that the internet gives everybody access to voicing their opinions, and I think it’s an incredible medium to communicate and to bring the world closer. But obviously, as an artist, or somebody who is creating something, you’ve gotta be careful how deep you dive into what everybody’s talking about, because it could really screw with your mind. I’ve always been in a place where I’m pretty thick-skinned, so it doesn’t bug me that much….

“It was difficult for Lou Reed because he takes everything very personally. And I think he was very surprised. We told him all along: ‘Listen, there are some very, very, very hardcore metal fans out there that like everything pre-packaged in a particular little box that looks like this, and the minute that you slightly veer outside of that, then they have a hernia.’